Turbofan engines are frequently employed in aviation. Referring now to the prior art design shown in FIG. 1 a typical turbofan engine 30 is illustrated. The turbofan engine 30 includes a case 32 surrounding a turbofan 34 and a number of compressor stages. Fan blade(s) 36 are secured to a shaft 38 by way of a rotor disk or hub 40 during normal operation. Conventional turbofan engines employ fan blade(s) 36 that are not integral to the rotor disk 40. Instead, the fan blade(s) 36 are individually joined to the rotor disk 40 by dovetail joints. The rotor disk 40 has mounting slots arranged around an exterior surface thereof. During normal operation, the shaft 38 rotates thereby rotating the hub 40. The hub 40 in turn produces the rotation of the fan blade(s) 36 around the shaft 38.
Referring still to FIG. 1, the turbofan engine 30 takes air into the engine at the turbofan 34 stage. The turbofan engine 30 directs the air through a number of low pressure, intermediate pressure, and high-pressure compressor and turbine stages before the air is mixed with fuel, combusted, and passed through a number of turbine stages. The air eventually exits the engine 30 through an exhaust. The various stages, like the turbofan 34 are configured to rotate with the shaft 38.
Air is pulled into the engine 30 through a front opening 42 of the engine 30 and into the turbofan 34 stage. The turbofan engine 30 may use conventional fan blades 36 that attach to the rotor disk 40 at a dovetail joint, as described above. However, the turbofan engine 30 may instead use an integrally bladed rotor or bladed disk (“blisk”). Referring to FIG. 2, a blisk 48 comprising a single component is depicted. Blisks may be machined from a single piece of metal, forged or cast as one part, or welded together into a single piece. In fabricating the blisk 48, fan blades or airfoils 50 are integrally formed to a blisk hub 52.
For the conventional turbofan 34 blade platforms 46 provide a secondary surface that has aerodynamic qualities surrounding the hub 40, which otherwise would have joints, mounting slots, and blade roots 44 exposed to the airflow through the turbofan 34 stage. Likewise, for the blisk turbofan 48 design, blade platforms 46 are not included, once again because the blisk fan airfoil(s) 50 attach directly to the blisk hub 52. Therefore, an analogous aerodynamic interior surface for the blisk 48 turbofan design is provided by the outer surface 54 of the blisk hub 52.
To provide a smooth flow of air through the front opening 42 of the turbofan engine 30, and into the turbofan 34, a nose cone assembly 56 is attached to the hub 40 or blisk hub 52. The nose cone assembly 56 has a generally conical shape. The central axis of the nose cone assembly 56 is substantially aligned with the axis of the rotating shaft 38. A conical tip 58 of the nose cone assembly 56 points out and away from the front opening 42 of the turbofan engine 30.
Opposite the conical tip 58, the nose cone assembly 56 expands to form a substantially circular base 60 that meets and is secured to the hub 40 or blisk hub 52 typically by a separate mount ring. The diameter of the substantially circular base 60 of the nose cone assembly 56 may be more or less large, relative the hub 40 or blisk hub 52, depending on the particular nose cone assembly design 56 and the manner by which the substantially circular base 60 is attached to the hub 40 or blisk hub 52. It is an objective of the nose cone assembly 56 to provide an aerodynamic path for air entering the front opening 42 of the turbofan engine 30.
Referring now to FIG. 3, the conventional nose cone assembly 56 utilizes an aluminum nose cone mount ring 62. The aluminum nose cone mount ring 62 of the conventional design also serves as a forward blade retainer 74 that secures the blade root 44 within a respective mounting slot. The nose cone mount ring 62 includes a securing component 76 such as bolts, screws, or other base end fastener pieces 70 that secure the nose cone mount ring 62 to a hub flange 72. These securing components 76 operate in conjunction with fingers or tangs extending radially inward from the nose cone mount ring 62 to attach to the fan disc hub 40. The fingers or tangs and the securing components 76 operate together to attach to the nose cone assembly 56. Moreover, a portion of the mount ring 62 radially outside the hub flange 72 acts as the forward blade retainer 74. The securing component(s) 76 used to attach the nose cone mount ring 62 to the hub flange 72 frequently serve the dual purpose of customizing forward balance and/or trim balance. Although the holes or other removal of material that provides space for the securing component(s) 76 may be used to customize forward balance. Conventionally an aluminum, composite, or polymer material is used for the nose cone assembly 56 to achieve reduced weight and cost. However, mechanically bolting to a polymer or composite material may cause stresses the selected material is not be capable of withstanding.
However, the blisk turbofan, being one integral component, obviates the need for the forward blade retainer 74. The nose cone assembly 56 for the blisk 48 turbofan may be attached by either a blisk nose cone mount ring or integral blisk attachment fingers 80. Referring next to FIG. 4, a configuration of the blisk 48 turbofan for use with a blisk mount ring is shown. The conventional blisk mount ring design may be used with the blisk 48 such that the mount ring 62 provides fingers, tangs, or a simple circumferentially continuous flange for attachment to the nose cone assembly 56 in a similar manner to the nose cone assembly 56 for the conventional turbofan 34 design. Alternatively, the integral blisk style may employ the fingers 80, tangs, or a circumferentially continuous flange attachment from the blisk hub 52 itself.
In order to accommodate the nose cone assembly 56, a lower arm 64 and flange 66 of the blisk hub 52 may be extended forward to provide a surface whereon a nose cone assembly 56 would attach. In the absence of an additional mounting ring, the flange 66 may have disposed thereon the integral fingers 80 fabricated as part of the blisk hub 52 and arranged about the extended flange 66. Forward extension of the lower arm 64 and the flange 66 may introduce significant difficulty in machining a cavity 68 thereabove. In addition, such a modification would extend the forging envelope forward a significant amount. The integral fingers 80 in this conventional blisk nose cone assembly 56 also provide trim balance. Further, just as above described with respect to the mount ring 62 configuration, forward balance is manipulated at the location where the nose cone assembly 56 attaches to the fingers 80 either by modification of the securing component(s) 76 or the material extricated to accommodate same.
As a result of the integral fingers providing trim balance for the blisk 48 and the securing component(s) 76 that connect to the integral fingers 80 through the nose cone assembly 56 or spaces thereabout providing forward balance, an interdependence between trim and blisk forward balance features is developed. A precise balancing may be difficult to achieve given the space constraints on configuring both types of balancing features along substantially the same plane of the nose cone assembly 56 inner circumference. Such balancing may be particularly difficult when considering inner flow path diameters having considerably smaller diameters derived by either small turbofan size, generally, or from a fan having a small hub/tip ratio.
Extension of the forging envelope may lead to further difficulties in machining the entire blisk, thereby increasing time of manufacture, weight, and overall expense. Adaptation of the blisk 48 design depicted in FIG. 4 produces a blisk forward balance problem. Moreover, any balance feature added further forward than the extent of the blisk 48 shown in FIG. 4, such as fingers 80 extending forward therefrom, becomes relatively less capable of providing balance adjustments because of the decreased radius of such component as compared with a trim balance feature located proximal the diameter of the blisk hub 52.
A need exists for better securing nose cone assemblies to blisk turbofan engines.